1. Field of the Invention
This invention relates, in general, to the field of ink jet printing mechanisms and, more specifically, to systems to control the build up of ink aerosol, that is, the build up of ink droplets in suspension, which are produced during operation in large format ink jet printers, plotters and the like.
2. Description of the Related Art
An ink jet device is a printing device without impact that forms characters and other images by ejecting ink drops in a controllable way from a printhead. The ink jet printing mechanisms may be used in different devices such as printers, plotters, facsimiles, copiers and the like. For the sake of convenience, in what follows reference will be made only to large format ink jet printers to illustrate the concepts of the present invention.
The printhead of a machine of the kind mentioned ejects ink through multiple nozzles in the form of minuscule drops which "fly" for a small space and strike a printing support. Different nozzles are used for different colors. Ink jet printers usually print within a range of 180 to 2400 or more dots per inch. The ink drops are dried upon the printing support soon after being deposited to form the desired printed images.
There are several types of ink jet printheads including, for example, thermal printheads and piezoelectric printheads. By way of example, in a thermal ink jet printhead the ink drops are ejected from individual nozzles by localized heating. Each of the nozzles has a small heating element. An electric current is made to pass through the element to heat it. This causes a tiny volume of ink to be heated and vaporized instantaneously by the heating element. Upon being vaporized, the ink is ejected through the nozzle. An exciter circuit is connected to individual heating elements to supply the energy impulses and, in this manner, to deposit in a controlled way ink drops from associated individual nozzles. These exciter circuits respond to character generators and other imaging circuits to activate selected nozzles of the printhead in order to form the desired images on the printing support.
As those skilled in the art know, during the operation of an ink jet printer of the mentioned class, the machine generates an ink fog constituted by very small ink particles (of a size usually less than 5 microns) that are stopped in their flight from the printhead to the printing support (within 1 mm from the printhead) and remain in suspension in the air. This fog of ink particles, also called "ink aerosol", builds up within the printer and is associated with undesirable deposition of said ink particles on critical surfaces of the printer (optical sensors, bearing surfaces, electronic assemblies, etc.), which may lead to failures, for example in the head position codifying system due to the presence of ink deposits on the encoder strip.
Moreover, the deposit of ink particles from the said aerosol upon surfaces such as the machine's windows, covers and doors, as well as upon other surfaces that may transfer ink onto the hands of the user or even onto the walls and floor of the room in which the machine is situated, may not only be annoying when handling the machine, but also the dissipation of said deposit into the atmosphere may be harmful from the environmental point of view.
This problem is aggravated in the case of large format, high performance machines (provided with bigger print heads having a great number of nozzles), in which the amount of ink used throughout the machine's useful life generates a considerable amount of aerosol.
In the document EP-0705700 an ink jet printing mechanism is disclosed, which includes: a plurality of ink jet printheads each one in a controlled way ejecting multiple ink droplets; a carriage which transports the printhead through a printing zone to a service station where at least one of the nozzles ejects in a controlled way ink droplets during maintenance mode; a collecting container (spittoon) situated in the service station to collect the ejected ink droplets; and multiple channels situated adjoining the container, with different channels receiving ink droplets ejected from nozzles of two different printheads and guiding said ink droplets ejected from the different printheads to reservoir of the container in order to avoid the mixing of different inks in said channels.
In no part of the above document reference is made to the control or elimination of the aerosol formed by ink droplets in the printing zone during the machine's operation. The document tries to address the problem that arises from the generation of droplets in suspension solely in the area of the service station of the said printhead nozzles. Nor it is indicated that said control is carried out by means of a flow of cleaning air.
In document EP-0568256 there is disclosed a use of an air flow and a filter for collecting and trapping the ink particles that are maintained in suspension during the operation of an ink jet printer. However, this known system is not usable except in low performance ink jet printers and its use is not satisfactory in the case of high performance, large format ink jet printers or plotters, with large printheads of the type mentioned hereinbefore, as with this system the ink droplets in suspension are collected, exclusively, in the machine's printing zone.
In a HP DesignJet 2000 CP Series printer, a fan is installed at the service station end of the printer, which is permanently running when the printer is operating, to generate a constant air flow in the service station area. However, no constant air path is available either in the print zone or in the service station zone. In fact, since the air flow is not designed to clean the print zone, the carriage is a first obstacle to the air path in the printing zone, i. e. the air path is varying depending on the actual position of the carriage.
Furthermore, the service station itself is a second obstacle to the air path in the service zone; in fact the service station, when it is moved in its servicing position, is substantially reducing the air flow in the service zone.
Finally, other methods already known in the art consist of modifying the formulation of the inks used or the design of the printheads, or in generating electrostatic fields in the printing zone to direct the ink particles in suspension towards the print medium. The applicant considers that all these have shortcomings from the economic point of view and due to their technical complexity, and/or they have a more negative impact on the design of the machine than the system in accordance with the invention.